Why are even drone flights prohibited due to the earthquakes near Mt Keilir, Iceland?
The 5th of Mar NOTAM (valid to 1st of June)...
(A0100/21 NOTAMN
Q) BIRD/QWWXX/IV/NBO/W/000/999/6558N01748W999
A) BIRD
B) 2103051657 C) 2106011657
E) DUE TO POSSIBLE VOLCANIC ERUPTION ON REYKJANES, THE ICELANDIC
TRANSPORT AUTHORITY, IN COOPERATION WITH ISAVIA ANS, HAS DECIDED ON
A PLAN TO BE FOLLOWED TO MAINTAIN SAFETY OF FLIGHT WITHIN THE AREA.
FOR FURTHER INFORMATION SEE AIC A002/2021
REF AIP ICELAND ENR 5.2
F) SFC G) UNL)

... references AIC A002/2021. Which notes:

Pilots are urged to use extreme caution due to possible heavy traffic and risk of mid-air collision.
Expect drone flights below 120 meters. [emphasis mine]

That does not agree with the tweet, though permission is required to fly:

Contact Keflavík Tower for permission to enter or pass through the area.


Also, AIP GEN 3.5 (GEN 3.5.3.9) notes:

During volcanic eruption, a mobile radiosonde station can be deployed near the active volcano. Also it can be deployed for research and training purposes. Weather balloons with radiosonde will be launched by IMO scientists. Prior to launch, authorization must be given by Reykjavik ACC. When the launching site is near an aerodrome, further authorization must be given by the respective AFIS/ATC services. Information on mobile radiosonde launches shall be published in NOTAM.