No, it was not maintained as a whole; after the withdrawal of pay to the army and possibly of many of the remaining troops, there was no central authority that was particularly interested in the wall or maintaining it. As the Roman province of Britannia gently slides into a patchwork of petty kingdoms, there is little point maintaining or manning a very large, very expensive structure that likely stretches across multiple domains. The wall is generally now assumed to have functioned more as a beacon of Roman power and as a customs barrier and early warning structure then anything else, which would have made it of very little use to the new elite forming in sub-Roman Britain.

What it is for and who built it is forgotten to the point where Gildas (writing some time between 490 and 550) presents us with a very different story of it to that of modern historians. This is almost certainly partly due to De Excidio Britanniae being a sermon and not a work of history and mostly concerned with how awful Britons have brought God's wrath down upon themselves, despite being frequently mined for historical detail, but it does indicate that a well educated man at that point either didn't know or care about the wall's "real" origins. He believes it was built for defence - as one would assume on looking at it - and that suits his purposes. Bede, whose monastery at Jarrow is practically on the wall, writing in approx. 730 follows Gildas in this.

However: that doesn't mean that some forts along the wall did not continue to be lived in and maintained, with evident continuity of occupation even as the style of that occupation changed. I would particularly point at Birdoswold, which shows evidence that people continued to live there until the ninth century. Although the styles of building changed (unsurprisingly over several centuries with huge cultural change) and some parts such as the baths become used for other purposes, they continue to be aligned along the same patterns, indicating that the same people were living there through to the ninth century.

Finally: as the wall has never been the border with Scotland (the modern border is further north, and early medieval borders tended to run north-south in line with the natural geography of the Pennines and 'invasions' from the east), there was little point having it as a "defensive feature". However, during the period when the border is unstable during the 14th-17th centuries there are some references to a night's watch using parts of it inside the debateable lands as a look-out position for reivers coming from the north. There is also some joking during Elizabeth I's reign about repairing and remanning the wall, but it was not ever taken seriously. Its only recorded contributions to military campaigns, as far as I know, since the "end of Roman Britain" are the battle of Heavenfield in the 630s, where an army from Gwynned which had been occupying York was pinned between the Wall and a crag by the Northumbrians, and the use of substantial chunks of it to build a military road (the modern B6138) from 1751 following the Jacobite rising of 1745.

For further reading, I'd suggest Richard Hingley's Hadrian's Wall: A Life which is extremely readable, relatively recent, and addresses the after-life of the wall in much deeper depth then I've gone into here.