The Brad Stevens era has been notable by just how unfamiliar most of it's players have been with the FT line. Outside of Isaiah Thomas, for much of his tenure I think most of his players viewed the FT line as some sort of abstract artwork painted on the floor, whose meaning and purpose they could not grok.
If not for IT4's two season averaging 6.6 & 8.5 FTA/gm, bringing the team totals to 23.5 & 23.2 FTA/gm in those years, in all other years the team barely managed to take just 20 FTA/gm, including a Stevens-era low of juts 19.5 per game last season (2nd last in the NBA).
This year, though, with Kemba, Gordon & the Two Jays, the team bounced back to average 23.2 per game. Still well behind the 25-28 per game we used to see when Paul Pierce was in his prime, but a huge improvement and good enough to rank 15th in the NBA. And what I like the most is that it was a team effort. Multiple guys getting 'a few' trips to the line each game (or a different guy getting a lot in any given game).
I personally think both Jaylen & Jayson are capable of averaging even more FTAs/gm, even within the shared USG model they are working in. They just need more experience (and reputation) at drawing fouls.