For the record, flashing custom roms to phones is not as popular as it used to be. OEMs have made their phones much harder and very risky to root and flash, with much more strict proprietary security tech that will brick the phone if one should try and mess up. It is a thing only done by real techys these days, that understand how and also are willing to risk bricking their expensive device.
As the major OEM phone models continue to advance, they are becoming less and less the target of custom roms. Less and less devs take up the initiative to develop. When it does happen, it often takes a very long time before anything stable and fairly easy is available to flash, even back years ago(i think I waited just over 1.5 years before any root and 1 rom became available for the LG flagship model I had, by then, the thing was old and outdated tech and no oneone stepped up to continue work on it because everyone had moved on to newer devices).
The real market for custom rom lay in off brand or less known here in the USA brands, OnePlus, and other Chinese phones seem to have the main openness for custom rom. In fact, most of the custom rom devs are not in USA, but are in other countries. The rest of the world may have openly embraced Custom roms…but the USA market…I doubt they will ever will.
Personally, having been a custom rom wizard(and part time dev), and not being able to live with out the perks that came with them…back in the day…I have found that current android, 8 and 9, I have no need for root or custom roms. The OS, (Google’s Stock experience anyway) is excellent. I have seen how features that were invented by community XDA devs and only available in their own custom roms, now are built right into the core OS. And I no longer have to have the random headache that are bugs or odd quirks that come with any custom rom.
Since, it seems, that RW is interested in hearing about why folks like us are attracted to custom roms, aside from things you already mention, like more control over privacy and security, and possibly getting security and os updates past the OEMs lifecycle of the device, here are my main reasons I used to swear by to use a custom rom:
Ad block. System wide. (no longer needed for me since all my apps are ad free paid versions, or open source no ads included version/Opera web browser have very effective ad blocker)
Viper4Android (or other system wide sound mod/enhancement) this was a big one. I am a very picky audio guy with very particular audio/EQ requirements. I switched to Poweramp V3 which has the best EQ there is, no root or mod needed. Sounds better than even when a system sound mod was enabled. I also invested in a good USB DAC, so now I can take my preferred audio experience to any Android phone.
Black theme. I do not understand the more modern trend in deleting the 16 million color palette and shadowing/shading design elements, to just all flat white and lots more whitespace. It is bad on the eyes, eats battery, and puts very talented graphic designers/artists out of work. (a “Dark” theme is now built into Android. Most OEMs also have their own stock theming settings, dark theme included)
More advanced power and hardware controls. A necessary thing, for a rom that is mass produced for many phone models. Google, as akin to Apple, is the only true OEM that is able to exclusively optimize their OS ROM for their own exact devices…and boy, does it show. My 3 yr old Pixel XL is still faster, and buttery smooth with little to no “jank” than my coworkers 1.3yr old Samsung phone. I have no need to fuss with a cpu governor or tweak frequencies or hunt down software timers, triggers, and wakelocks…the phone and its OS just works buttery smooth, and gets me days of battery life…and has yet to slow down or degrade after 1.5 yrs, as every single other phone has on me.
Customization. Here is where things are a bit poor. Many custom roms have a HUGE amount of customization available to the user, some, too much so. Part of me is glad that I don’t have to spend days trying to tweak every single of the million things to make it just perfect to my liking. But, there are things I wish that I could tweak or customize about the UI on my Pixel that do kinda bug me and would make my life a tiny bit easier if I could. But, the amount of stock customization is just sufficient, with the basics I need are there.