If you type:
"/effect @p " (with the space after 'p')
and press tab, it will come up with "minecraft:".
Press tab again and it will come up "minecraft:strength" (or whatever effect) and in the chat will list all possible effects. Keep pressing tab to scroll through all the available effects, but "clear" will never show up.
Linked issues
Comments 4
Why don't you want to fix, I mean you could either add clear to the list of potion effect names, or you could change it, that it reads a list for other arguments!?
Searge never thought about a category like "minor bugs" or "get fixed in far far future"?
Note: This bug and another bug that I posted, MC-66922, are greatly linked and should both be seen together.