Hasn't always been like this though
Per Cleaning the Glass, top 2 in limiting corner threes 5 of the 7 seasons prior to 19-20 and never outside the top 10.
I don't subscribe to Cleaning the Glass
al I can do is go thru BBREF. They say
2013-14: Blazers 1st in rate of opponent corner three's....17th in opponent corner 3pt%
2014-15: Blazers 5th in rate of opponent corner three's....8th in opponent corner 3pt%
2015-16: Blazers 7th in rate of opponent corner three's....8th in opponent corner 3pt%
2016-17: Blazers 2nd in rate of opponent corner three's....8th in opponent corner 3pt%
2017-18: Blazers 3rd in rate of opponent corner three's....22nd in opponent corner 3pt%
2018-19: Blazers 2nd in rate of opponent corner three's....9th in opponent corner 3pt%
2019-20: Blazers 21st in rate of opponent corner three's....20th in opponent corner 3pt%
2020-21: Blazers 24th in rate of opponent corner three's....30th in opponent corner 3pt%
so, it looks like bbref and cleaning the glass have similar stats
What separates the fist 6 seasons from the last 2 seasons in terms of defending corner three's? I'd say it's definitely linked to personnel.
in the first two season, the Blazers had Aldridge, Matthews, and Batum as starters. All were good at rotating to open three point shooters. Those teams also had 2nd unit players like Dorell Wright, Will Barton, Earl Watson, Steve Blake, and Arron Afflalo who were at least average to pretty good at perimeter defense
the next 4 seasons had Aminu and Harkless as the starting forwards. And 2nd unit players included Gerald Henderson, Noah Vonleh, Tim Frazier, Evan Turner, Shabazz Napier, Pat Connaughton, Seth Curry. Again, no lockdown defenders, but a sizable collection of at least average to good defenders
but before last season, for some reason (drugs?), Olshey decided that defense didn't matter. He dumped a lot of the decent perimeter defenders in favor of Bazemore, Hejonja, & Tolliver at the same time he cleared the way for Simons and Trent to have big roles. Then he signed Melo. That's kind of a killer's row of defensive screen doors. Olshey tried to put his finger in the dyke last offseason by bringing in RoCo and Jones. But with Dame & CJ already marked for major minutes, there are a lot of supporting minutes devoted to crappy defenders like Trent (31.2 minutes), Melo (25.6), Hood (19,2) and Simons (17.0). Those 4 guys are worse defensively than just about any of the perimeter players on those previous 6 teams