You have to be really careful to distinguish between the position that the canon is temporarily, functionally closed and that it is closed permanently. You can definitely find plenty of people who support the strict position, but I believe that it is less popular than the looser position overall, especially when looking outside of Christian apologetics circles.

There's a few good reasons to think that the canon is only temporarily closed, not permanently closed:

1. The Bible wasn't canonized or seen as a single book until after Revelation was written, so it is unlikely that John had the whole Bible in mind.
2. Revelation says that the restriction is on "the book of this prophecy", i.e., the book of Revelation itself. Even if you correctly consider that "prophecy" is more than just foretelling, there are parts of the Bible that don't count as that.
3. If you read them carefully, you'll see that Deuteronomy and Proverbs do not say anything against saying God's words in a different way or recontextuallizing them to apply them to a different situation. The problem only comes about if you change the meaning of the message.
4. At least according to both Claude and GPT, the idea of a strict closure didn't take root until the Reformation (about 1.5 millennia later).
5. A non-strict interpretation fits better with the fact that the story of the Bible is not yet finished. If the story is unfinished then it's likely that God will do more works which ought to be recorded. For example, it would probably be helpful to the people living through the great tribulation to know what the actual history was that led up to that event.