What I expected to happen was...:
A long message in chat usually has hanging indention just like in 20w16a and previous snapshots and versions.
What actually happened was...:
The message no longer has hanging indention, and it can eventually become unpleasant if there is many of them.
How to reproduce:
Type a long message in chat.
→ You will notice that the long message is no longer indented.
I can confirm that long messages are no longer indented in 20w17a.