Was Danelaw still a thing by the time Normans came? If not, was there any efforts to reconquer it?

For all of the attention it's given, the Danelaw is actually a fairly shortlived phenomenon, at least in terms of being a distinct polity, and the treaty itself may have been normative rather than actual. Certainly by the 890s, Æthelred of Mercia was granting land in charters that was theoretically in the Danelaw, indicating either that he had little intention of respecting it, or that the English felt sufficiently secure to disregard it.

The Danelaw itself was largely recaptured between around 900 and 924 by Alfred of Wessex's children Edward 'the Elder' and Æthelflæd who, having married Æthelred of Mercia at some point in the early 880s, ruled the kingdom independently following his death in 911 after the Battle of Tettenhall. The Danelaw is an active threat to the Mercian frontier, at least until its heavy defeat at Tettenhall. Æthelflæd spends the next half decade or so significantly reforming and improving the military and defensive infrastructure of Mercia before going on the offensive in 916, storming Derby. In 917 she secures Leicester, before campaigning against York in 918, securing the city's surrender. To the East, Edward leads West Saxon forces in a similar progressive assault, capturing and fortifying, Essex, East Anglia, and Lincolnshire. When Edward dies in 924, his son Æthelstan inherits a kingdom which has physical control of everything south of the Humber, and at least token control of everything North of it.

Maintaining that control and establishing a cultural hegemony is a hard task. Control even of the Midlands, let alone Northumbria, remains occasionally problematic until the 940s, with a brief wave of attacks on Tamworth which are eventually defeated when York is captured militarily in 954. It should be noted at this juncture that Northumbria had been a major rival of Mercia during the 7th, 8th and early 9th centuries, and so it was likely to resist any attempt to bring it into line with a unified England, before Danish settlement is even brought into the equation. Northumbria remains politically troublesome until the reign of Edgar in the 970s, who mollifies the region by essentially granting it functional autonomy in return for its allegiance and taxation, essentially operating it as a satellite kingdom.

Culturally, the Danes in East Anglia and Northumbria appear to assimilate fairly rapidly into the new English polity. The archaeological record shows that they were very quick to adopt English fashions, particularly in pottery and personal wear metalwork such as strap-ends, with West Saxon 'Trewhiddle' designs proving popular. Vice versa, Danish influence also impacts English material culture, with York in particular becoming a centre of strap-end production featuring a distinctive 'Borre' style which eventually becomes fashionable across England. Interestingly, it's Mercia rather than the Danelaw which is the artefactual outlyer, with Mercia remaining adamantly culturally aceramic beyond state-mandated production well into the mid-10th century, ironic considering Stoke-on-Trent and Stafford's reputation as a site of international significance in pottery.

The ethnographic divide appears to have lingered somewhat more perniciously; Edgar is forced to intervene for example after some Anglo-Danish merchants from Northumbria are harrassed in Rochester, with the port city fined for threatening to disrupt the national peace. Far more catastrophic is the St Brice's Day Massacre of 1002, carried out on the orders of Æthelred II, who saw ethnic cleansing as a convenient way to deal with the potential problem of Anglo-Danes supporting Norwegian attacks on England. Of course, the massacre sparked international outrage and guaranteed an escalation of conflict.

By the time of the Norman Conquest, the Danelaw hadn't existed in a real political sense for over a century, but the more-heavily Anglo-Scandinavian Northumbria was however still largely de facto autonomous, if not independent. This was arguably, however, as much a result of Anglian Northumbrian independence as it was Danish, as the Anglo-Danish areas of East Anglia and the East Midlands remained essentially compliant.