The annual trail closings are why SoBos are advised to not plan start dates earlier than June 1st.
Statement from the Park:
All Katahdin and Traveler trails are now closed at their trailheads to protect alpine resources. Alpine plant communities are particularly vulnerable during the shoulder season freeze-thaw cycles. Rangers will evaluate resource conditions on a regular basis to assess when these trails may open.
Togue and Matagamon gates are closed to all vehicular traffic. The winter camping season has ended and the summer camping season is anticipated to begin May 15. Day use access is possible from outside the gates. Please stay on trails during this period while trails remain wet and muddy.
The shoulder season is a time for the natural resources of the Park to rest.
If the pattern of earlier seasons hold, trails should re-open in late May.
More info: The 2021 SoBo's Guide to Baxter and Katahdin.