If the warranty has expired... it means that at least two years have passed in which you have used it, one presumes.
Defective or not, the warranty has expired.... and is therefore out of warranty... otherwise what would be the point of having a warranty if it is over.... then you can always try the manufacturer... never say never.
In any case, an inflated battery, especially a lithium-ion battery, can be dangerous and, if punctured or comes into contact with air, can cause fires and explosions. Using the powerbank or leaving it connected to the power supply can pose significant risks. Act immediately to ensure your safety and the safety of those around you.
The powerbank may cause fire and explosion, posing a significant danger. The internal pressure of the battery increases as it inflates, which may cause it to rupture or damage and then burst/explode/fire. Inflated batteries should not be ignored or underestimated, as they can cause serious damage if handled improperly.
To ensure your safety and the safety of those around you, act immediately when dealing with an inflated battery. Dispose of it properly according to the manufacturer's guidelines. Avoid puncturing or tampering with an inflated battery. Do not use and/or charge it.

When LiIon batteries start to swell, that's the end of it. Dispose of correctly and that's it. There is no longer a guarantee and repair is not possible.