I'm personally of the opinion that I really think how they handled Hollyhock here was very effective. It is upsetting and frustrating, but that's a bit of the point. You don't need to see what's on the letter or see Hollyhock again, all the pieces for what happened and why it happened are all in the show very clearly even without saying it directly. There's enough context to explain what happened, why it happened and what it means to Bojack, and I think it even makes a lot of sense.
Furthermore, the fact that what Hollyhock does can start a dialect among viewers (I've seen much back-and-forth both here on Era, Reddit and a Discord I'm in on Hollyhock) and left an impression on a lot of people, yet how the show honestly handles it is very quietly, shows to me it was an impacting scene for many (even if some are really upset by it or can't agree at all with what Hollyhock does), and it shows a certain type of thing that does happen in life sometimes. Sometimes people cut you out of their life because you've done shitty things or they just can't be comfortable or close with you anymore, and you get no real closure with them, things just kinda' end. It does happen like that sometimes, and sometimes it's even very clearly the consequence of your choices in the past that leads to that. One of the stronger themes of the back-half of Season 6 in my opinion is that sometimes there's irreparable damage you've done and things you can never get back because of it, and I think what happens with Hollyhock personifies that in a realistic way perfectly.
And it fuels a stronger element of Bojack's own character arc, losing Hollyhock makes him spiral for a bit, but it also makes him more seriously reflect on himself and even though it hurts him a lot, life still goes on. Some things have changed forever for him, but other things are still pretty much the same.
