The Broncos used the 20th pick of the 2017 draft to take an offensive tackle - Garett Bolles, who was the first tackle drafted that year. And he has finally started to play like a first-round talent.
Prior to that, the Broncos had not spent much draft capital on tackles or other offensive line positions. The Broncos now have not drafted an offensive tackle since Bolles (unless you count Dalton Risner, who has not played a snap at tackle in the NFL).
That means that there have been 103 offensive tackles drafted since the Broncos last took one.