The thing with creative constraints is that they're invitations, not mandates.
For example, I think that the classic 140-character limit on Twitter of old is an amazing creative restraint, and I even added a switch to my toot front-end to set that as an optional limit.
I think it's totally fine for a writer to challenge others to write without the use of rich text formatting. It's up to the other to decide whether or not that is a valuable constraint. The same can be said for post limits (500 characters drives me nuts, honestly), extensive CSS formatting, posting in absolute plain text, or whatever.
Heck, it's equally valid to say THEANCIENTGREEKSALLWROTELIKETHISSOWHYARENTYOU!
We could totally do a #NoSpaceNoCapitalizationNoPunctuationChallenge if we wanted to! XD
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