“We’re aware of reports that access to Signal has been blocked in some countries,” Signal says. If you are affected by the blocks, the company recommends turning on its censorship circumvention feature. (NetBlocks reports that this feature lets Signal “remain usable” in Russia.)
Threema still works.
IIRC, Threema’s crypto algo is a patchwork cluster of copypasta and prayers.
When choosing a crypto algorithm the answer is almost never “roll your own”.