Since update 2.0, there are player level thresholds for item Tiers, meaning that after reaching a certain level, you unlock a new vendor stock and loot Tier.
Weapons
Tier 1 from Level 1
Tier 2 from Level 9
Tier 3 from Level 17
Tier 4 from Level 25
Tier 5 from Level 33
Tier 5++ from Level 51 (Phantom Liberty-only)
Cyberware
Tier 1 from Level 1
Tier 2 from Level 10
Tier 3 from Level 20
Tier 4 from Level 30
Tier 5 from Level 40
Note that reaching a given level threshold means that items of this Tier become widely available, but you can still encounter weapons of lower or higher Tiers.
Moreover, each Tier has a Tier+ variant, which is an improvement from the non-plus variant without an additional level requirement.