I've only found information about printers or brands (like Canon) known to cause problems with Ubuntu.
Since my parents asked me to buy them a printer now (and I myself have an old Canon printer (!)), I don't know which brand or model to choose that will work like a charm.
They only want to print very occasionally, so I've read a small b&w laser printer would be best for them. They just want something to plug in and print. No fuss.
Can you recommend a brand or simple laser printers that you know work with Ubuntu 20.04 without any special driver installation? (I'm keeping the LTS on their laptop as long as it is supported, so 20.04 will probably stay there for a while.)
Thank you for your help.
Let's refer you to openprinting.org's printer database
There you can search for what works and what doesn't.
Any modern printer (one you buy in the shops) will work with Ubuntu. Absolutely guaranteed. I'll refund your purchase price if it does not.