Quick answer
Most aren't, but a few are. Easy levadas (Balcões, Fanal, Levada do Rei) are safe walking paths. The vertigo-exposed routes, Caldeirão do Inferno, Levada do Caldeirão Verde tunnel sections, PR1, have unprotected drops, narrow ledges and pitch-dark tunnels. Bring a headlamp, check the live closure list, and skip them in heavy rain.
Context
Levadas were built as 200-year-old irrigation channels, so they follow contour lines and don't climb steeply. The hazard isn't elevation gain, it's the unguarded downhill edge, which on some routes is a multi-hundred-metre drop.
Fatalities are rare but happen most years, usually from slipping on wet rock, walking off the path in fog, or trying to swim under waterfalls. Almost every incident is preventable with appropriate gear and weather judgement.
Always carry a headlamp (tunnels), wear grippy shoes (not sandals), and check the live walkability page before driving to a trailhead.