Gas heating required in property purchase contract

I am curious to hear what the notary has to say about this passage.

Especially since I expect progress in photovoltaic and storage possibilities in the next few years.

That was also the case during our planning and construction period. Everyone expected "progress in the next few years." By now, about 9 years later, hardly anything has really changed.

So I wouldn’t let the property be spoiled because of that.
If everything else fits, do it and clarify it afterwards if in doubt.

I see it the same way. No big deal.

With gas and the option to switch (THAT I would check), you are definitely better off than those who have such a compulsion for district heating or similar with monopoly prices and minimum consumption, which unfortunately is also permitted.

If you have concerns, then insulate very well and without calculation tricks. Photovoltaics always make sense (still), for heating it is not that relevant.

You can later also use a second heat source like a heat pump and decide yourself how much gas you use and when. However, that only makes sense/economical later on when gas is as out of fashion as it is today too expensive to heat directly with electricity.

In general, if there is the possibility to use gas, I would also rather opt for a condensing boiler than an air-water heat pump or similar.

I find it somewhat alarming that there is a connection obligation in the gas sector and am not sure if it is possible – this must at least be recorded in the development plan and thus also tolerated by the municipality.

Two points are crucial:
Just because you have to connect and use the connection does not mean you have to get the gas from the municipal utilities. You have free choice if you do not like the municipal utilities in general or the price.
However, it will certainly also be the case that you have covered a large part of the gas infrastructure costs through the purchase of the property. Surely enough has been allocated there.

No idea if this is even permissible, especially indefinitely.

But I wonder, who is the seller? The city?

The rights from the purchase contract first have to be exercised by someone. Unless this obligation is recorded in the land register in favor of the municipal utilities, I wonder how they intend to enforce their alleged right?

In case of violation of this right, it would also need to be clarified what damage occurs. Court proceedings and such. Are penalty clauses recorded in the contract?
No one will probably try to challenge the purchase contract over such nonsense.

By now, about 9 years later, hardly anything has happened.

Parallel universe?!

Photovoltaics today cost about 25% of the investment compared to 9 years ago.
This can also be seen in the Renewable Energy Act remuneration. In 2008, it was still over 46 cents/kWh, today's new systems 11,x cents/kWh.
Besides the cheaper investment, the performance per module has also drastically increased, so that a reasonable generator capacity can be installed on a single-family home.

Storage systems 9 years ago were more in the realm of nerd territory. Today they are widely available and have more than enough capacity to serve single-family and multi-family homes (without occupying entire rooms). Every electrician offers them and commissions them.
However, something still needs to happen regarding economic viability.

That is partially permissible. A special reason must be present. You might want to follow up on that if necessary. It is not necessarily uncommon either. Google "Baugebiet Gasversorgung Pflicht" there is an interesting article from the Bundestag's website where all the necessary information is included.