i am seeing subtext blocks used a bit more frequently when i'm looking through various chats, though it mostly seems by a handful of active users (not trying to say anybody is bad for using it, just that it's being used because it's a newer, getting-to-be-known feature and fun for a lot of people).
even though i don't have proper vision concerns like some other users present, it's rough enough for me to try and read that i've just started to unfortunately toggle the ignore feature (before block was the only option) on people who are using it heavily. i've needed to start to just reveal their messages if the conversation seems engaging enough to break my eyes/mind while trying to read their part in it. this action path doesn't make me feel good though, because i value their input and voice! 😔
discord has ways of scaling up text, but that is the only alternative to avoid the formatting downsize... while making all other text extra large beyond what you want as normal readable text. and leaves the mid-grey tone of the text an issue for readability depending on theme selection.
it would be nice if discord gave us an option to disable viewing of markup, or at least accessibility options to disable certain markup. unfortunately nothing seems to exist to counter this from a user side. but in the meantime i am reviving this a bit to see if people have changed their feelings on this or have anything to add. 
additionally, subtext has been used to fake system messages to less informed people, and is a "minor" security concern in some senses because of that. this is one reason why some other servers regex automod messages with subtext, aside from accessibility concerns for users with various degrees of vision acuity.
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