Is it supported?

No, not at this time. There are some backwards incompatible issues that may cause problems if you try to use PHP 8.0

I run my site via Docker, and didn't specify the version of the PHP-FPM image I wanted to use and during a server migration I couldn't get past this error:

www_1      | 2020/12/23 17:46:29 [error] 35#35: *9 FastCGI sent in stderr: "PHP message: PHP Fatal error:  Maximum execution time of 30 seconds exceeded in /var/www/brfcs/mb/system/3rd_party/JShrink/Minifier.php on line 546" while reading response header from upstream, client: 194.35.116.206, server: tugay.brfcs.com, request: "GET /mb/ HTTP/2.0", upstream: "fastcgi://172.19.0.3:9000", host: "tugay.brfcs.com"

Turns out JSShrink has issues with PHP 8 at the moment ...

 

No, not at this time. There are some backwards incompatible issues that may cause problems if you try to use PHP 8.0

Right

I have tried to upload a theme manually but it give me an error.

Just let you know, when you're working on Php 8, think also upload a theme