我做了一些实验后解决了这个问题。
验证码：

let bool = false;

function showMore() {
  var textContainer = document.getElementById("textContainer");
  var blurContainer = document.getElementById("blur");
  var showMoreButton = document.getElementById("showMoreButton");
  
  if(bool == false) {
    bool = true; 
    textContainer.style.height = "auto";
    blurContainer.style.display = "none";
    showMoreButton.innerHTML = "Show less";
  } else {
    bool = false;
    textContainer.style.height = "25vh";
    blurContainer.style.display = "inline";
    showMoreButton.innerHTML = "Show more";
  }
}

body {
  display: flex;
  justify-content: center;
  overflow: auto;
  margin: 50px 0 50px 0;
}

.container {
  position: absolute; 
  width: 25vw;
  height: 25vh;
  border: 1px solid black;
  overflow: hidden; 
}

.container p {
  padding: 10px;
  text-align: justify; 
}

.gradient-blur {
  display: flex;
  justify-content: center;
  position: absolute; 
  padding: unset;
  bottom: 0; 
  height: 50%; 
  width: 100%;
}

.gradient-blur > div,
.gradient-blur::before,
.gradient-blur::after {
  position: absolute;
  inset: 0;
}
.gradient-blur::before {
  content: "";
  z-index: 1;
  backdrop-filter: blur(0.2px);
  mask: linear-gradient(
    to bottom,
    rgba(0, 0, 0, 0) 0%,
    rgba(0, 0, 0, 1) 16.5%
  );
}
.gradient-blur > div:nth-of-type(1) {
  z-index: 2;
  backdrop-filter: blur(0.4px);
  mask: linear-gradient(
    to bottom,
    rgba(0, 0, 0, 0) 16.5%,
    rgba(0, 0, 0, 1) 33%
  );
}
.gradient-blur > div:nth-of-type(2) {
  z-index: 3;
  backdrop-filter: blur(0.8px);
  mask: linear-gradient(
    to bottom,
    rgba(0, 0, 0, 0) 33%,
    rgba(0, 0, 0, 1) 49.5%
  );
}
.gradient-blur > div:nth-of-type(3) {
  z-index: 4;
  backdrop-filter: blur(1.6px);
  mask: linear-gradient(
    to bottom,
    rgba(0, 0, 0, 0) 49.5%,
    rgba(0, 0, 0, 1) 66%
  );
}
.gradient-blur::after {
  content: "";
  z-index: 8;
  backdrop-filter: blur(2.2px);
  mask: linear-gradient(
    to bottom,
    rgba(0, 0, 0, 0) 66%,
    rgba(0, 0, 0, 1) 100%
  );
}

#showMoreButton {
  position: absolute;
  bottom: 5px; 
  height: 25px;
  width: 25%;
  color: #386FE5;
  z-index: 9999;
  cursor: pointer;
}

<div class="container" id="textContainer">
    <p>
    Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed  diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.   

Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi. Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat.   

Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat. Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse
  </p>
  <div class="gradient-blur" id="blur">
    <div></div>
    <div></div>
    <div></div>
  </div> 
</div>
<button type="button" id="showMoreButton" onclick="showMore()"> Show more </button>

以完整视图打开代码段以正确查看所有内容。

使用background-image属性作为掩码可能不是所有浏览器都支持的。

.gradient-blur {
position: relative;
width: 100%;
height: 100%;
backdrop-filter: blur(10px);
opacity: 1;
background-image: linear-gradient(transparent, black 75%); }