phpBB was quite nice, but you must remember that people used phpBB less and less over the years. Many phpBB style webforums are dead, and died before discourse etc... came about.
People's habits changed.
I do agree that things got worse in the last ~16 years or so.
What really killed phpBB and that generation of boards was mostly the sketchy codebases they ran off.
The code was rife with vulnerabilities, so the boards needed constant patching (which was a non-trivial that sometimes killed the database). If you didn't patch on time, a script inevitably dropped by, exploited the software, dumped all credentials, and nuked the database.
Those old forums were not built with the adversarial nature of the 2010s internet in mind. Boards were dropping like flies a few years there. Most simply never recovered.
Absolutely avoid all the extensions. Supposedly that got tightened up in v3.x but I saw some boards get pwned in 2.x from the extensions. Another issue is that most people were too lazy to harden php.ini yup this is a thing and their servers allowed outbound connections so exploiting some of the core code was much easier. Maybe I am just lucky but I never had a security issue with phpBB. One of my earliest forums using phpBB had over 50k people on it. That may not sound like much but it was a niche community and very early in the Internets existence.
The legal landscape has also changed. 20 years ago I helped run a web forum, but with today's legal landscape - DMCA in the US, various different laws in the EU and other countries - I would never do so. The amount of liability on the host for user-created content is far too high.
It did change a lot but the biggest changes were the ToS/AuP of server/VM providers. What was not even taboo in the early 00's was becoming a problem keeping an account active on clear-web sites. Across the board many providers starting using the vague word "lewd" a word I had never heard of even after running porn sites for a long time.
Many of us moved to .onion despite being incredibly slow at the time. We would keep an unpublished clear-web sub-domain active for the old time users so they had a fast connection. Eventually that was even problematic so many forum operators stopped accepting new users and made their forums private or semi-private. Some still exist and some got married, had kids and real life took too much time and energy to also run such sites.
Europe is more bureaucratic. You have to register with the government to say you host user content and respond to abuse reports, and then respond to abuse reports, and you're shielded.
Compliance certification costs money - lots of it. The laws also require proactive scanning of uploaded material, not just responding to complaints, and require a person to be declared liable for mistakes.
Gone are the says of paying a couple hundred a year for a host and a vBulletin license. The new costs are many thousands plus permanent liability, and no thanks - that's way more than I'm going to do for a group of enthusiasts online.
There's one or two still kicking around that I visit myself, but I'll admit I don't miss not having threaded conversations. Trying to follow a conversation with other people butting in with low effort shit posting is way harder with everything being linear.
I still use it for private and semi-private forums. The access controls make it much easier to stop drive-by spammers or at very least prevent anyone seeing their crap posts.
For phpBB site behavior, access control lists, ranks unless discourse has added that, per board policies, per rank policies. I have not used Discourse in a long time so I have no idea what they have added but that was the difference long ago.
People's habits changed.
I do agree that things got worse in the last ~16 years or so.