Classical Chinese had a much larger phonemic inventory than modern Mandarin, and notably no tones. Below are a collection of Classical Chinese reconstructions in IPA that are all pronounced yì in Mandarin today. (like "ee" for English speakers). The creation of tones and other sound changes were fairly predictable, so as you say, the hints often still help today.
Nit point (I'm not sure it's relevant), but we don't know to what degree Old Chinese did or dit not have tones. The very first work to say anything at all about pronunciation is a Middle Chinese text from ~600AD, which already did have a system of 4 tones, albeit a different 4-tone system than Mandarin. Old Chinese pronunciation is a reconstruction from very limited data, not unlike proto-Indo-European, despite being considerably closer to the present.
I just looked it up and the phonetic markers are only like 20-30% reliable. I am shocked at this number as in my experience I would have thought it higher (I would have guessed 60-70%), but it is definitely hit-or-miss. I've never found tones to be predictable.
- ŋjajs 議; 'discuss' - ŋjət 仡; 'powerful' - ʔjup 邑; 'city' - ʔjək 億; '100 million' - ʔjəks 意; 'thought' - ʔjek 益; 'increase' - ʔjik 抑; 'press down' - jak 弈; 'Go' - ljit 逸; 'flee' - ljək 翼; 'wing' - ljek 易; 'change' - ljeks 易; 'easy' - slek 蜴; 'lizard'