I think your question needs clarification
Did you have a negative test?
If negative , you don't have HIV.
If you had the antibodies, they would still be detectable. However, it would be rare to have HIV for 20 years and not get AIDS, the disease. However, rarely, this has happened.
If you have not been tested and are worried, it is still worth testing.
If you had a negative test, and your risk is "negative" except for the exposure 20 years ago, then you don't have HIV.
If you were exposed within the 20 years and have not been tested, you should still get the test.
Seroconversion would have occurred years ago, but you simply might not have become ill.
Seroconversion would not take more than 6 months, and in most cases, not more than 3 months.
There was one reported case years ago of sero conversion after 18 months, the longest on record.
I am a double board certified use doctor and clinician. I actually am trained in HIV and take care of HIV patients, and have for over 20 years.