seeker wrote:

Has anyone learned of a method to trace back ozone levels or how thick
the ozone layer was at various times in the past?

Only over the period where measurement devices for UV absorption have
been available. The classic measurement device was invented by Dobson
around 1920 and measurements of ozone column depth still bear his name.

Although it is possible to take an educated guess that the ozone layer
was thinned a bit in the two years immediately after Krakatoa. Displays
of polar stratospheric clouds were seen over a wide range of latitudes
couple that with large injections of material including HCl into the
stratosphere and conditions would have been right for ozone destruction.

I don't think anyone was able to measure this damage at the time, but
there may be some serendipitous observational records somewhere. Farm
workers suffering unusual sunburn and that kind of thing.

Regards,
Martin Brown