My understanding is its generally someone with the (now very anachronistic) approach to football of going to see their local team play matches, preferably at a time when they are taking a break from the working week (this resulted in the obviously ridiculous idea of games being played of a weekend, thankfully long consigned to history). They are often just the latest of many in their family. They'll cheer when the team plays well, but sadly can be highly intolerant at perceived lack of effort. They are also known to sing unpalatable songs at players and opposition 'legacy fans'. This used to be a working class thing and most legacy fans were regular attendees and lived nearby, but that's increasingly less so these days, particularly among the 6 'elite' clubs that rotate between the 1st and 9th positions.
Historically they created the culture and funded the success of the club, however the primary function of a legacy fan in recent years is to provide an exciting soundtrack to the more important 'customers' who watch the excitement from their front room (or local hostelry for the more 'ultra' types).
This arrangement has been problematic for the clubs as these legacy fans are often unreasonable in their demands, and owners have the inconvenience of actually facing them on match days (should they elect to visit the stadia at all), however covid has thankfully provided a solution. By creatively piping a soundtrack into the tv boxes and iPads* the need for legacy fans has been mercifully reduced. This helps the elite clubs greatly as 'customers' are generally more pliable, gullible and willing to pay staggering amounts of money to drape themselves in the finery of the club while never going anywhere near the place, or if they do its generally a shopping trip.
*the 'iPad' customers are a particularly new phenomenon and have difficulty co-existing with the legacy fans as they are often subjected to requests to demonstrate such iPads and then further subjected to vile abuse.
