I think that would be difficult to implement
well in SMF, and I'd be reluctant to implement it
poorly.
With many people not using quote tags properly, or not using them at all, a functionality like this could lead to missed notifications if one were to rely on it. We could try and implement something along the lines of "if a quote tag is used and it explicitly references a post ID that belongs to you, we notify you in some way", but things would quickly get messy if we tried to encompass the different ways in which people engage with the forum.
I think your best bet is to use the "
new replies" functionality. This would normally track replies to any threads you're active in (and with Parsfial's modifications you can subscribe/unsubscribe from threads). It's not exactly what you're asking for, but at least it can be expected to be reliable.