I just had a go with a Pi5 and RaspberryOS, and no issues to report at all!
I just downloaded the .deb ARM package of https://www.cendio.com/thinlinc/download/ and installed it normally with $ sudo dpkg -i ./thinlinc-client_4.16.0-3389_armhf.deb
Unrelated, but I experienced severe mouse lag (locally) on the Pi5. This was mitigated by adding usbhid.mousepoll=0 to the end of /boot/firmware/cmdline.txt and rebooting the Pi5.