Sure, you can run any new circuit you want to an older panel. The panel does not care. In fact, the only restrictions would be for aluminum. Some terminations are not rated for AL.

The problem is with solid aluminum wire for 15 A and 20 A branch circuits.

The softness (lack of temper) required to make the wire flexible for installation makes it soft enough to deform under connections when it heats up (even a little) and this makes the connections become loose.

This creates more heat, deformation, and can lead to arcing and fires.

The purple caps are better than standard wire nuts.
The problem appears to be that to function the spring in the purple wire nut becomes a part of the current path and is simply not sized for that function.

If the Al conductors are well cleaned to bare aluminum and the purple nuts installed immediately (Aluminum oxide starts forming instantly on the clean surface) with solid contact between the conductors the inhibitor does slow the oxide growth, but it does not last forever.

The Copalum crimps require training before the manufacturers will sell them to anyone.

The problem does not occur with larger stranded aluminum conductors since they are suitable tempered. The bend radius is already large, based on the total conductor size, so the flexibility needed for single solid wires is not required.

It is the larger ones (central air, electric heat, water heaters, stoves, dryers. etc.) that should be fine if they are stranded aluminum.

Just as in the past large size copper wire is painfully expensive compared to (even larger for the same ampacity) stranded aluminum.