There are many "amateur" weather sites, and some of them do carry out of date data.
A good example is
this one which carries the genuine UKMO MSLP charts but if you look closely on the age of each one, you will see that some of them are older than the "same" ones on say Avbrief (or perhaps the UKMO's own site, but I don't have the pay subscription to check).
This is not unusual because nobody wants to pay money to the UKMO, so they often carry older charts, which amounts to a forecast which is further-out. It isn't wrong as such; it's just that a later forecast for a given date/time will be more accurate.
There are also sites which carry out of date tafs/metars, which is inexcusable since these are free. Most sites get them from
here, for free, anyway. Some more difficult TAFs (e.g. LGST) can be found
here (as well as historical TAFs, which are rare).
Nobody should rely on interpreted forecasts like "tomorrow will be a fine day"...