I have a mint SP1 Colt, but saw a Colt AR-15A2 for sale and it has the same goofy screw style upper.

are these colt compatible with a RLL?

A2 Sporter II, absolutely - drop in with factory paddle and no adjustments needed.  It and the SP1 are the guns for which the RLL was specifically designed.

One of the later Colt A2 named models might need sear block removal or clearance milling, and finding an SP1-spec bolt carrier.

Aren't all older AR15 rifle compatible with a RLL?

When did they start making the high shelves to prevent it, like mid 90s?

All are potentially compatible, but there have been carrier differences, and interior machining differences since day 1. Some builders only used surplus FA bolt carriers, for instance, which have never been RLL compatible.

It was probably 1990/1991 before Colt started with the sear blocks and shaved carriers, and then some makers followed suit with high shelves shortly afterward and it reached full retard after the 1994 AW ban.  But some guns were still faithful to the SP1 internal spec and SP1 semi carrier spec until the mid 2000s. Olympic Arms and Bushmaster (original Windham) and DPMS (original St. Cloud) were RLL compatible almost up until the sellout to Cerberus.

Yes I know about the sporter II. They are almost as much as SP1s now