I am having this problem as well! I ended up individually downloading all the icons for social media sites and remaking the social links in my footer with a code block with alt text. I had to add an inline css height attribute to make all the icons the same height and width. It looks more or less the same and it doesn't flag any errors for WAVE... but now you have to know HTML to edit it. Not a great trade off since I'm may have to pass the website off to someone less HTML-literate at some point, but works for me for now. I used this kind of code:
<a href="twitter.com/isfsd"><img style="height: 3em;" alt="Twitter" src="https://images.squarespace-cdn.com/content/5e4467780f0455f95cff518/1585073685193-GKZBXZ9EXWCXL2URQS29/Twitter_Social_Icon_Square_White.png?content-type=image%2Fpng"></a>
Can you access Markdown blocks? I know lower-tier plans don't have code blocks, but I tested this code in a Markdown block and it worked there as well.