The whole point of running an opensource OS/vm/runtime/etc is that you can tweak things here and there when you need them.
Yeah, we had to make changes, some changes to FreeBSD too, but mostly little changes here and there. Erlang and FreeBSD were both lovely to work on.
Re the sibling's question about what was changed, I don't remember everything, Rick Reed's presentations at Erlang Factory / Elixir describe most of them though (although those ended in 2014, I think). Many or most of the changes got into upstream one way or another. But most of it were things because AFAIK, we had much larger Erlang clusters than the rest of the community; I remember seeing advice about large clusters of 50 when we were running 300 nodes in a cluster, and I'm pretty sure we had dist clusters above 1000 later when we also had separate cross cluster messaging. We also had huge mnesia tables, other people said don't use mnesia over 2GB, and we had nodes with more than 512GB of data in mnesia. I don't really remember much that we had to do with dist, although pg2 needed help and our replacement became pg in OTP, mnesia did need some help to scale. We changed the ETS hash kernel to avoid everything hashing the same way, I don't know if that made it out.
We also needed to do things like timer wheel improvements, but OTP also did timer wheel improvements and we dropped ours. Not so many people were running quite so many timers.
Then there were things that I don't think are upstreamable. Adding a way to drop a process's message queue. Adding a way to add a message to the front of a process's message queue. Those two are very not in line with OTP, but handy for operations if you use them carefully.
Process groups, some solid performance tweaks to ETS and Mnesia, etc. It all got upstreamed or became redundant eventually. Rick Reed gave a couple talks about it around a decade ago if you can track down recordings.
