The Lifetime ISA has existed for 2 years. In each of those years the annual limit has been £4000 and the bonus has been fixed at 25% in each period too.
The government could change both the limit and the bonus in the future but have announced no plans to automatically index link either. ISAs have existed for much longer and their limits have changed over time, those limits have also never been explicitly index linked, they change (or not) whenever there is a budget.
A treasury select committe has even called for the abolition of Lifetime ISAs saying their complexity is too high and their uptake rate too low, so there's no guarantee they will even exist in the long term.