Regulations do not address such technical requirements. The authorities only put general guidelines in the regulations, e.g. the plant must be sufficiently secured against expected faults.

The standards are used as an aid to achieve the authorities’ requirements, but they are not binding. They describe only a recognised method of achieving sufficient safety as described in the regulations. This is “managed” by the industry, but is also accepted by the authorities. If you don't wish to comply with the norms, you will be responsible for documenting that the solution is good enough to satisfy the regulations, which are binding.

For heating cables, please take a look at the following norm points:
IEC/EN 61892–7:2007 clause 7.21, and
IEC/EN 60079-30-2:2007 section 10.1c) and 10.3.1.1 (found in NEK420:2010)
These all state that repairing heating cables MUST be in accordance with the supplier’s descriptions. This is to ensure that the heating cable’s certification as Ex equipment will still apply. Due to the many technical solutions on the market, it would be impossible to create a general rule, other than “follow the manufacturer’s instructions”.
Where it may be stated in IEC 61892 that the heating cable cannot be spliced with an approved splice kit/coupling box is unknown to me.

In order to return to your original question, repairing damage using a heat-shrinkable sleeve with adhesive or vulcanising tape will only be permitted if this repair method is described in the manufacturer’s documentation. All methods described are part of the approval and therefore do not violate the requirements of the certificate, the standards, or the authorities' requirements in the regulations for that matter.