Currently ff_rosters() will return latest week but by adding an additional parameter end user can select specific week:

sucioboys <- ffscrapr::espn_connect(season = 2020, league_id = 899513)
ffscrapr::espn_getendpoint(sucioboys, view = "mRoster")
#> <ESPN - GET https://fantasy.espn.com/apis/v3/games/ffl/seasons/2020/segments/0/leagues/899513?view=mRoster>
#> List of 8
#> $ draftDetail :List of 2
#> $ gameId : int 1
#> $ id : int 899513
#> $ scoringPeriodId: int 18
#> $ seasonId : int 2020
#> $ segmentId : int 0
#> $ status :List of 21
#> $ teams :List of 10
ffscrapr::espn_getendpoint(sucioboys, view = "mRoster", scoringPeriodId = 1)
#> <ESPN - GET https://fantasy.espn.com/apis/v3/games/ffl/seasons/2020/segments/0/leagues/899513?view=mRoster&scoringPeriodId=1>
#> List of 8
#> $ draftDetail :List of 2
#> $ gameId : int 1
#> $ id : int 899513
#> $ scoringPeriodId: int 1
#> $ seasonId : int 2020
#> $ segmentId : int 0
#> $ status :List of 21
#> $ teams :List of 10