I think the letter is rather formal but not unreasonable.
I think many of the replies on here are ascribing thoughts and intentions to these PILs which are not clear from this letter.
There is so much unknown here.
I can imagine another interpretation where the OP is hard work as a DIL (there is some evidence to support this interpretation on the other thread) and therefore PILs are closer to other DIL and children just because they are easier to be around. OP takes offence realtionships deteriorate further, words are had, MIL says 'maybe easier not be in contact.'
OP holds tight to this statement and decides to go NC. PILs feel on reflection some effort should be made to maintain a cordial relationship at least, and take steps through FIL writing letters, asking them to be part of family events and meet up., and stating they are always open to a relationship.
This is an interpreation. I don't know the truth. (whats' truth?)
All we have OPs opinions of PILS and some evidence from the letters which from responses on here may/may not be reasonable.
People making sweeping statements about how toxic they are and what their intentions are is rather dangerous I feel.
Step back, give advice cautiously when you don't know the facts. consider other interpretations, things are not always as simple as bad people/good people.
OP you and your Dh know the context. I would seek measured advice from people who know you and who have your best interests at heart.
Posting your PILs letters on the internet for sensationalist responses, may at this point be comforting in that they reinforce your position and feelings, but may really not be best for you and your family in the long run.