Code that demands to pay off its dept isn't non-working code, it's rather code that exceeds one's ability to maintain it properly (your mention of "unreadability" included). Whether a PR introduces debt isn't always known and often times has to be discovered later on, depending on how fast its maintainers fluctuate and the ecosystem advances.

That said, tech debt isn't paid by developers individually, it's paid by organizations in developers time. Only in rare cases can you make a deliberate decision for it, as it grows organically within any project. For example, most python2 code today that used niche libraries with outdated docs that have been taken offline in the meantime has to be considered expensive tech debt nowadays.