Must soccer fields be flat, and if not what is the maximum allowed slope from end to end and side to side?

Dimensions of a pitch aren't set in stone, but they all agree that a flat and level pitch is needed so the ball control isn't affected. However, when playing on a turf pitch, I've found a few referrences to the recommendation of a 1 - 2% slope radiating from centre to corners to allow drainage. For example:
http://www.livestrong.com/article/405109-soccer-field-construction-specs/

But according to this webpage, the FiFA recommendation is less than 0.5% slope (max. of 1% slope).
https://journals.sagepub.com/doi/full/10.1177/0020294016629177

Before recent renovations/rebuilding, the home ground at West Bromwich Albion used to drop by over a metre, and the one at Yeovil Town used to drop by 6 feet. Both of them were legal (back then).

West Brom's Balancing Act - 19th July 2011

A West Brom spokesman said: "During the close season, we have undertaken a number of works at the Hawthorns, one of which has involved the levelling of the pitch as previously it fell by over a metre from North to South. "Unfortunately, in order to achieve this, we have had to raise (we were unable to lower it) the pitch in front of the Smethick End stand by over a metre, resulting in the loss of two rows of seats across the whole length of the stand, in both the home and away sections. "The seats were lost as they would have been too far below pitch level to provide a satisfactory view of the match."

http://redsaway.com/2011/07/west-broms-balancing-act/

Sloping Off - Dec 2002

Yeovil regularly attract crowds that most Second Division sides would be proud of. And in 1990, well before it became fashionable, the Glovers re­loc­ated to a new out-of-town ground, Huish Park. But the club's fame is largely a con­sequence of regular Cup runs - since their formation in 1895 Yeovil have beat­en a record 20 League sides - and the old sloping pitch, an almost unplayable six feet from side to side.

http://www.wsc.co.uk/the-archive/30-Clubs/2738-sloping-off

Until 2013, Barnet FC played their home fixtures at Underhill Stadium, where the north end of the pitch was eight feet (sometimes 11 ft) higher than the south end. When the wind is also blowing from the north, the side defending the south goal is at a definite disadvantage.

http://reckless-guide-to-barnet.blogspot.com/2014/04/underhill.html