There are 125 rule changes for 2021. Many of them are just wording changes to increase clarity. There are multiple changes regarding moving timing issues to a consistent point in time before the next serve occurs.

Official 2021 Rulebook (pdf)

2021 Rules Change Document (pdf)
This document lists all of the rule changes, the reasons for the change, and has scenarios that could occur during a match.

Major Changes for Serves

• Let serves have been eliminated. Serves that hit the net and land good are played.
• The drop serve is added as a provisional rule. Provisional means that USA Pickleball will evaluate the rule for unintended or unexpected outcomes over the next year. Players may also continue to use the current serve. See the video for an explanation and demonstration.