Since the Captains started being announced in 2007, this is only the second year where the Patriots have started the season without their starting QB being named as a Captain. The other was in 2016, where Brady was suspended the first 4 games. Brady was eventually named Captain in November that year, so it's possible that Mac Jones will eventually earn the title later into the season. If he does not, it'll be the first complete season the Patriots have had without their starting QB as a Captain since they've been announced. Cam Newton was named a Captain prior to the start of 2020 despite it being his first year on the team.
The 5 Captains named is the second lowest total the Patriots have had during that time, the lowest being the 2010 season when they only had 4 (Brady, Faulk, Wilfork, Mayo) and 2016 when they started with 4 (Gronk, Hightower, McCourty, Slater) but moved to 5 when Brady was named Captain later in the year. They also had 5 in 2009 and 2013. Though NFL guidelines say you can name up to 6, the Patriots have had up to 8 (2020, 2008) and have had 7 on multiple occasions (2019, 2017, 2015, 2007).
Slater especially continues his epic streak, with McCourty close behind. Both have been named Captains every year from 2011 on, with the lone exception of McCourty not being named a Captain in 2013. Slater's 10 consecutive years as a Captain easily tops Wilfork's 7 year streak from 2008-2014 and Mayo's streak from 2009-2015, and is the second longest stretch for a player in that time, as well as the second most total years for a player in that time. The longest streak and most total years being Brady of course. McCourty's 9 total years are similarly 3rd most of any player during the time Captains have been announced.
All 5 have been captains in 2019, 2020, and now 2021. This is only the second time 5 players have been Captains for 3 years in a row together. The first being 2011-2013, with Brady, Mankins, Wilfork, Mayo and Slater being Captains all 3 years. Mankins was not a Captain in 2014 so 3 years is the longest a group of 5 Captains have held that title.
Regardless, as indicated all 5 well deserve it, congrats to them!