Ok, let me just try to compare. Data points are the summer playoff rosters 2017 vs. 2019. I'll just try to bring up the data without further comment, as I'm kind of neutral on the whole "NA imports too much" topic. I'll list the imports by teams, and then summarize in the end. Also I'll show the "import-percentage" of each team. Source is lol.gamepedia.com.
Season 2017
C9 2017: Ray, Impact, Jensen. Ray and Impact shared toplane. (3 of 6 players were imports)
CLG 2017: Huhi (1 of 6 players imported)
EF 2017: Looper, Froggen, Gate. Gate was counted as NA resident (2/3 in 9 were imported)
Flyquest 2017: No Imports. 5 Players
Immortals 2017: Flame, Xmithie, CodySun, Olleh, Dodo, Unnamed Italian Player. Dodo, CodySun and Xmithie were NA residents. (3/6 in 7 players were imported)
Phoenix 1: Inori, Ryu, Pirean, Arrow. Inori was an NA resident. (3/4 in 10 were imported)
Dignitas: Ssumday, Chaser, Shrimp, Keane. Keane was an NA resident. (3/4 in 8 were imported).
Team EnvyUs: Seraph, Lira, Pirean, Nisqy. Seraph was an NA resident. (3/4 in 6 were imported)
Team Liquid: Reignover, Inori, Michey, Piglet, Konkwon. Inori and Konkwon were NA residents. (3/5 in 10 were imported).
Overall Season 2017: 21/30 in 67 players were imported. This gets us to an "import-percentage" of 31.3 % just counting non-NA residents. And an 44.7 % when also counting "not-NA nationals".
Season 2019
100 Thieves: Ssumday, Amazing, Ryu, Bang. Amazing was an NA resident. (3/4 of 7 were imported)
C9: Svenskeren, Nisqy. (2 in 8 were imported)
Clutch: Huni, Lira, CodySun. CodySun was an NA resident. (2/3 in 5 were imported)
CLG: Ruin, PoE, Biofrost. Biofrost was an NA resident. (2/3 in 5 were imported).
EF: Rush, Panda, Fenix, Lost. Panda was an NA resident (3/4 in 10 were imported)
FQ: V1per, Santorin, Wadid. V1per and Santorin were NA residents (1/3 in 6 were imported).
Golden Guardians: Froggen, FBI, Olleh, Huhi. Huhi was an NA resident. (3/4 in 7 were imported).
Optic Gaming: Crown, Scarlet, Arrow. (3 in 6 were imported).
Team Liquid: XMithie, Jensen, CoreJJ. Jensen and Xmithie were NA residents. (1/3 in 5 were imported).
TSM: BrokenBlade, Spica, Bjergsen, Zven. Spica and Bjergsen were NA residents. (2/4 in 7 were imported).
Overall Season 2019: 22/33 in 66 players were imported. This gets us to 33.3 % just counting non-Na residents. And an 50% value, counting "not-NA nationals".
So, as a final result, the import percentage rose slightly from 2017 to 2019, but not by much. 1 in 6 players in the League are "Naturalized" imports, that have gained NA residency.