The weapon tiers are an accumulative grade of some of the best players' top 33 weapon rankings. Any weapon not included in this tier list isn't viable or relevant enough to warrant a ranking.
The newest version of the tier list can be found in the wiki's official discord server, which you can join by clicking the module on the right hand side of this page, or use this link. There is a channel called "community-tier-lists," where you can find the latest version of the weapon tiers.
You can also contribute to the tiers by making your own personal tier list, which then has its scores added to the larger community tier list. Anyone over level 15 on the discord server can create their own tier list, and all tier lists are weighted equally and impartially. If you see something you disagree with in this list, feel free to create your own.