Bronze: Bronze level is the lowest sub loyalty level. This level includes all subs that have been subbed to the channel for more than 3 months. This is the lowest level to determine channel longevity and higher amount means either a brand new channel that is growing in popularity or a channel that has low higher sub loyalty.
Silver: Silver level is the second lowest sub loyaly level. This level includes all subs that have been subbed to the channel for more than 6 months. This level has the double duration of the previous level Bronze. This level is more important to show if a channel is keeping more people to sub to the channel for a longer duration.
Gold: Gold level is the middle sub loyatly level. This level includes all subs that have been subbed to the channel for more for than 1 year. This level is a great way to show channels with good amount of sub loyatly. Channels that has high level of this level means that the channel is stable and the stream is consistent to keep people subbing for long durations.
Platinum: Platinum is a high sub loyalty level. This level includes all subs that have been subbed to the channel for more than 2 years. This level shows that the channel has been streaming consistently for a long time and has the loyalty of their subs. High sub loyalty of this level indictive of a channel with healhty sub base.
Diamond: Diamond is currently the highest sub loyalty level. This level includes all subs that have been subbed to the channel for more than 4 years. This level can only be achieved by channels that have been on Twitch for a long time as getting subs for more than 48 months requires channel to be live and streaming for longer. Most of these are the oldest channels on Twitch and having high diamond level sub loyalty shows that the channel is very stable and popular for the sub base. The channel must be active for the last 4-5 years to keep the viewers subbed for more than 4 years.
We could add higher level sub loyalty levels but anything higher would be achievable only by really old channels that have been streaming on Twitch since the beginning.