Make sure you keep a copy of the photo of the obsolete sign and make sure it is dated.
A valid contract cannot be formed if there is confusion or ambiguity in the signage at the location. For a contract to be legally binding in the context of parking enforcement, several elements must be present and clear, unambiguous signage is a critical factor.
For a contract to be formed, there must be a clear offer (the terms on the signage) and acceptance (the driver parking under those terms). If the signage still references a previous operator, it creates confusion about who is making the offer and what terms apply.
The terms and conditions must be prominently displayed, legible, and clear enough for the average driver to understand. Ambiguity in the signage undermines this requirement.
The parking company issuing the Notice to Keeper (NtK) must demonstrate that they have the authority to manage the land and issue charges. If there is signage that references a different operator, it could cast doubt on whether the current operator has the legal authority to enforce parking terms.
Under consumer law, ambiguous or misleading terms are interpreted against the party that drafted them (in this case, the parking company). The presence of old signage is misleading to motorists and unfair under the Consumer Rights Act 2015.
Personally, I wouldn't bother with an IAS appeal but feel free to do so if you want to waste your time and effort. Others here will assist you. I would wait and see if they ever bother to try to issue a claim against you for this and let them waste their own time and money on issuing a claim that would not stand up in court.