fastnbt is used at run time in 18 crates (of which 2 optionally). It's used only as a dev dependency in 5 crates (of which 4 directly).

Depender (with downloads and own rev deps) fastnbt version
420 mc_schem ^2.4.4
410 3 fastanvil ^2
390 rustmatica ^2.5.0
330 minedmap ^2.3.2
280 1 mcdata ^2.5.0
fastnbt-tools ^2
1 minedmap-nbt ^2.4.4
lessanvil ^2.3.2
mca-cuboids ^2.2.0
mcatool ^2.1
1 cubic-protocol ^2.1.0
2 mca-parser ^2.3.2
mc_map2png ^2
player_mover ^2
1 bird-protocol optional ^2.2.0
cobble-core optional ^2.3
950 3 quartz_nbt dev ^2.4
750 12 simdnbt dev ^2.5.0
1 fastsnbt dev ^2
azalea-nbt dev ^2.4.4