According to the US CDC, if you are durably undetectable, meaning your viral load is undetectable for longer than six months, then you are not able to transmit HIV to anyone sexually.
Once you’ve been undetectable for a year or more, by the way, it is very highly unlikely that you will ever develop drug resistance or experience viral rebound.
There is a possibility, however, that if you had unprotected sex with somebody infected with a strain of HIV that was resistant to two of the drugs in your HAART regimen, that you could become co-infected.
If that were to happen, you could experience acute HIV infection all over again and become infectious. Your medication would need to be changed to get the new infection under control.
This is very, very unlikely, but it is remotely possible.
So, while you aren’t infectious, and having unprotected sex with a partner doesn’t present a risk to the partner, you do take on a very slight risk, yourself, by having unprotected sex.
Then there are the issues of consent. Even though the US CDC acknowledges that you don’t present a risk of infection, and even though it’s statistically safer to have unprotected sex with you than protected sex with a random stranger, social and legal awareness haven’t caught up to that reality.
So, those are some things to think about.
But in short, no, if you’ve been undetectable for 20 years, you’re unable to infect anyone with HIV.