Thunder hammers per se
certainly were post-heresy at least up until 3rd ed. – Chaos Terminators had to make do with their forerunners, power mauls.
Of course, that doesn't prevent you using 'power hammers' or similar, but if you wanted to be conservative in your interpretation, count them simply as power weapons rather than thunder hammers.
With all of these sorts of questions, I always err on the side of caution – while there probably were
Thunder Hammers or plasma cannons or whatever, they're not particularly associated with the pre-heresy era. By including them (or things that look like thunder hammers etc), you dilute the pre-heresy appearance and impact.
Basically, my approach is: if in doubt, leave it out.
I actually find these sorts of restrictions improve the feel of my army, as it forces me to try new things and approaches.
As a final thought, you should always do what you like – after all, screw the rules and do what's cool: there's always an explanation!