Does Teferi, Time Raveler's static ability prevent opponents from responding to their own spells?

Teferi, Time Raveler has a static ability that reads: 'Each opponent can cast spells only during their own turn.' This means opponents are restricted to casting spells only during their own main phases when the stack is empty — the normal window for sorcery-speed casting per CR 307.1.

Because opponents cannot cast spells at instant speed, they cannot respond to any spell or ability — including their own spells. Responding to a spell means placing something on the stack while that spell is already on the stack, which occurs at instant speed (CR 116.1, 116.2). Since opponents are forbidden from casting at that speed, they simply cannot respond.

This also prevents opponents from activating abilities with timing restrictions equivalent to instants (though activated abilities that aren't spells are not affected by Teferi unless separately restricted — note that Teferi only mentions 'spells,' so activated abilities may still be used at instant speed unless another effect stops them).

Concrete example: Your opponent casts Lightning Bolt targeting their own creature to save it from dying to your removal. While Lightning Bolt is on the stack, they cannot cast Redirect or any other instant to interact with the stack, because Teferi's ability prevents them from casting spells outside their own turn.

In summary, Teferi creates a hard lock on opponent spell-casting to sorcery speed (CR 307.1), which entirely eliminates their ability to respond to anything on the stack, including their own spells.