Considering that every online card transaction within Europe now requires the more complicated payment intents model (because of the SCA, Strong Customer Authentication, requirements) I would imagine that it's a fairly significant percentage.
Just as an anecdotal data point, this is not actually true in practice, at least not yet for us. We abandoned our considerable efforts to move to Stripe's new API during its catastrophic phase, for reasons I've mentioned in past HN discussions and won't rehash here. The bottom line is that anything that required 3D Secure would presumably now fail for us. However, as a UK business with customers in several other EU member states as well, we have yet to notice any significant failure rate when signing up new customers. This may be because we charge relatively low subscription fees and qualify for one or more of the exemptions most or all of the time. In any case, we keep being told it will change one of these days but so far the sky has not fallen and we're still using Stripe when taking card payments.
I presume its down to the bank. Most of them likely choose to avoid it if possible as it's a barrier to people using their card.
However, I use Monzo as my main current account. I would say that around 50% of my online payments send me through 3D secure, which for them involves pressing the approve button on a notification and entering my card PIN.
Just as an anecdotal data point, this is not actually true in practice, at least not yet for us. We abandoned our considerable efforts to move to Stripe's new API during its catastrophic phase, for reasons I've mentioned in past HN discussions and won't rehash here. The bottom line is that anything that required 3D Secure would presumably now fail for us. However, as a UK business with customers in several other EU member states as well, we have yet to notice any significant failure rate when signing up new customers. This may be because we charge relatively low subscription fees and qualify for one or more of the exemptions most or all of the time. In any case, we keep being told it will change one of these days but so far the sky has not fallen and we're still using Stripe when taking card payments.