Usando max-content dentro da função min()

// just a test button

button.onclick = function() {
  const sidebar = this.closest('.page').querySelector('.sidebar');
  if (this.classList.contains('active')) {
    sidebar.textContent = 'Lorem ipsum dolor sit amet, consectetur adipisicing elit. Perferendis, exercitationem?';
  } else {
    sidebar.textContent = 'Lorem ipsum';
  }
  this.classList.toggle('active');
}

* {
  margin: 0;
}

.page {
  display: grid;
  height: 100vh;
  grid-template-columns: fit-content(20%) 1fr;
}

.sidebar {
  background: yellow;
}

.main {
  background: blue;
}

<div class="page">
  <aside class="sidebar">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Perferendis, exercitationem?</aside>
  <main class="main">
    <button id="button">just a test button</button>
  </main>
</div>

// For demo purpsoses
const root = document.documentElement;
const main = document.querySelector("main");
const left = document.querySelector("aside");
const right = document.querySelector("article");

const ui = document.forms.ui;
const io = ui.elements;

io.view.value = `${io.left.value} ${io.right.value};`;

ui.onchange = (e) => {
  const chg = e.target;
  if (chg.name === "lr") {
    root.style.setProperty("--l", io.left.value);
    root.style.setProperty("--r", io.right.value);
    io.view.value = `${io.left.value} ${io.right.value};`;
  } else if (chg.id === "flex") {
    io.left.toggleAttribute("disabled");
    io.right.toggleAttribute("disabled");
    main.classList.toggle("flex");
    io.view.value = chg.checked ? "flex;" : `${io.left.value} ${io.right.value};`;
  }
};

/* For demo purposes */
:root {
  --l: 150px;
  --r: minmax(450px, 80vw);
  font: 2ch/1.5 "Segoe UI";
}

html,
body {
  width: 100%;
  height: 100%;
  margin: 0;
}

/* See the blue text for the value of grid-template-columns */
main {
  display: grid;
  grid-template-columns: var(--l) var(--r);
  grid-template-areas: "aside article";
  column-gap: 0.5rem;
  max-width: 100vw;
  margin: 0 auto;
  padding: 1rem 1.5rem;
}

/* The rest is for demo purposes */

.rel {
  position: relative;
}

aside {
  color: red;
}

form {
  position: absolute;
  bottom: -20vmin;
  left: 1rem;
  z-index: 1;
  background: #ddd;
}

fieldset {
  padding-bottom: 0.5rem;
}

select {
  font: inherit;
}

output {
  display: inline-block;
  margin: 0.5rem 0;
  color: blue;
}

#view {
  display: inline-block;
}

#view::before {
  content: "grid-template-columns: ";
  color: black;
}

.flex {
  display: flex;
}

.flex aside {
  flex: 1 1 auto;
}

.flex article {
  flex: 0 1 auto;
}

.flex #view {
  display: inline-block;
}

.flex #view::before {
  content: "Main display: ";
  color: black;
}

#auxl,
#auxr {
  display: none;
}

#auxl::before {
  content: "Left flex: ";
  color: black;
}

#auxr::before {
  content: "Right flex: ";
  color: black;
}

.flex #auxl,
.flex #auxr {
  display: inline-block;
}

<main class="rel">
 <aside>
  <p>No? Well, that's what you see at a toy store. And you must think you're in a toy store, because you're here shopping for an infant named Jeb.</p>
 </aside>
 <article>
  <p>The path of the righteous man is beset on all sides by the iniquities of the selfish and the tyranny of evil men. Blessed is he who, in the name of charity and good will, shepherds the weak through the valley of darkness, for he is truly his brother's keeper and the finder of lost children. And I will strike down upon thee with great vengeance and furious anger those who would attempt to poison and destroy My brothers. And you will know My name is the Lord when I lay My vengeance upon thee.</p>
 </article>
 <!-- The rest is for demo purposes -->
 <form id="ui">
  <fieldset>
   <output id="view"></output>&nbsp;
   <output id="auxl">1 1 auto</output>&nbsp;
   <output id="auxr">0 1 auto</output><br>
   <label>Left&nbsp;
    <select id="left" name="lr">
     <option value="minmax(150px, 20vw)">minmax(150px, 20vw)</option>
     <option value="minmax(20vw, max-content)">minmax(20vw, max-content)</option>
     <option value="minmax(min(150px, 20vw), max-content)">minmax(min(150px, 20vw), max-content)</option>
     <option value="max-content">max-content</option>
     <option value="20vw">20vw</option>
    </select>
   </label>
   <label>&nbsp;Right&nbsp;
    <select id="right" name="lr">
     <option value="minmax(450px, 80vw)">minmax(450px, 80vw)</option>
     <option value="minmax(80vw, max-content)">minmax(80vw, max-content)</option>
     <option value="max-content">max-content</option>
     <option value="1fr">1fr</option>
     <option value="80vw">80vw</option>
    </select>
   </label>
   <label>&nbsp;flex&nbsp;
    <input id="flex" type="checkbox">
   </label>
  </fieldset>
 </form>
</main>

.parent {
  width: 100vw;
  height: 100vh;
  display: grid;
  grid-template-columns: minmax(min-content, 20vw) 1fr;
}


.sidebar {
  background-color: lightblue;
}


.content {
  background-color: lightgreen;
}

<div class="parent">
  <div class="sidebar">
    SIDEBAR CONTENT GOES BRRRRRRRRR
  </div>
  <div class="content">
    MAIN CONTENT
  </div>
</div>

.parent {
  width: 100vw;
  height: 100vh;
  display: grid;
  grid-template-columns: minmax(auto, 20vw) 1fr;
}


.sidebar {
  background-color: lightblue;
}


.content {
  background-color: lightgreen;
}

<div class="parent">
  <div class="sidebar">
    SIDEBAR CONTENT GOES BRRRRRRRRR
  </div>
  <div class="content">
    MAIN CONTENT
  </div>
</div>