Pressing ctrl-shift-u followed by some hexadecimal digits types a unicode character with the given code-point, for instance ctrl-shift-u 2266 gives ≦. Strangely, some unicode code points fail in my configuration (mostly fresh install of Ubuntu 20.04), for instance typing ctrl-shift-u 2264 inserts nothing (instead of ≤), in the same way that pressing the compose key and some inexistent combination inserts nothing.
Testing all numbers starting at 2200, I found that the following fail: 2219 (∙) 221a (√) 2248 (≈) 2264 (≤) 2265 (≥) 2320 (⌠) 2321 (⌡) 2580 (▀) 2584 (▄) 2588 (█) 258c (▌) 2590 (▐) 2591 (░) 2592 (▒) 2593 (▓) 25a0 (■). Actually, I found the first handful by accident, then came across the KOI8-R encoding, which seems to contain precisely these characters that do not work for me. There is nothing Russian about my install (bought the computer new, in France, and I have not selected Russian locale or anything like that).
While I don't routinely type such characters directly using ctrl-shift-u, they also fail with the compose key, specifically typing Compose > = inserts nothing instead of ≥.
