Short version:
New in Microsoft Teams on mobile: optimised network data usage setting. Select an option to reduce the bandwidth used in video calls and meetings. Settings > Data and storage > Reduce data usage. This feature rollout has begun and should be complete by mid-March 2021.

Details:
This feature update will allow you to select an option to reduce the bandwidth used in Teams video calls and meetings. Click Settings > Data and storage > Reduce data usage to select if you’d like to reduce the data used in the following circumstances:

  • Never – default
  • Cellular
  • Always

With this feature enabled, Microsoft will deliver the best quality experience for the given meeting complexity (number of video participants, variety of bandwidth conditions, multiple modalities, etc.) limited to 500 kbps on average.

Availability:
This feature rollout has begun (I have just checked and I already have this) and should be complete by mid-March 2021.