Update to "How To Add Nonstandard Tuplets" How To Page
I have MuseScore 188.8.131.5288, and the location of the splitting measures feature is different. For me, it's in Tools -> Measure -> Split Measure Before Selected Note/Rest.
Same goes for joining measures. For me, it's in Tools -> Measure -> Join Selected Measures.
I think it would be a good idea to update the Howto page on this topic for those who have the latest version.