By coincidence I just asked myself the same question and made it my holidays project. I installed El Capitan on my mid-2014 MBP (it does support Big Sur but it

By coincidence I just asked myself the same question and made it my holidays project. I installed El Capitan on my mid-2014 MBP (it does support Big Sur but it was unbearably slow & laggy and battery runtime was not good), and it seems a 2010 mac mini should be capable to run El Capitan too. Why El Capitan? Because the most current Chrome requires El Capitan (Mavericks does not work). Actually I use ungoogled-chromium. While it might seem super dangerous to connect to the internet because of security, I took some measures: On my Router (Fritzbox, but any other quality router will do): - Isolate that device to "Guest" WiFi so it can only connect to the internet, not other devices in my LAN (works by Mac Address Filtering) - Do not allow unknown MAC Addresses to connect to WiFi at all for all my home network - Only allow outgoing ports 80/443 TCP and 53 UDP on the Mac, refuse all incoming connections (its NAT anyway but just to be sure) - Disable UPnP for that device When using the device: - Never use ANY login/private data on that device (see exceptions below) - Never enter any private data, so no social media, email, iCloud etc. just plain surfing - I strictly only use AirDrop from my modern Mac or a readonly SDcard to copy files ONTO it (Movies, Music, Ebooks, Podcasts), never download or copy the other way around. - The only Applications I installed are ungoogled-chromium (plus ublock origin) and VLC I use it to watch (offline) movies, music podcasts and sometimes Netflix. Additionally I connect my E-Piano with a piano learning website to it over USB (Benefit is I can use GarageBand instrument library with it). For those two websites the passwords are strictly unique. When not in use, I always do a full shutdown (no sleep). I know that a super-dedicated and driven hacker might - with lots of work - use the device as attack vector, but I mostly think this is a highly unlikely event. Especially since it is way easier to compromise servers with log4shell right now :) Edit: Btw it is super easy to downgrade, just hold CMD+Option+Shift+R on powerup, and the internet recovery will install the macOS version the device shipped with. Then go to https://support.apple.com/en-us/HT211683 https://support.apple.com/en-us/HT211683 and download the full OS as a simple .pkg file on another machine, copy to SDcard and start installation.