Are teams allowed to wear face shields instead of safety glasses. Face shields are less likely to fog up when wearing a mask and would seem to offer the same or better protection.
Yes, this would be permissible, provided that the chosen face covering falls within a given event's Health & Safety guidelines, per <G1>:
For the 2020-2021 season, some events may establish additional Health & Safety guidelines beyond the scope of this Game Manual. These guidelines will be communicated to all Teams in advance via Health & Safety notes associated with the event registration in RobotEvents. All Teams (including Students or any Adults associated with the Team) must abide by these guidelines as written. Violation of an event-specific Health & Safety rule may be considered a violation of <G1> and/or the REC Foundation Code of Conduct.