I have a bicycle that's locked with a 2 cm/0.8" hardened steel lock that I have unfortunately lost the key to. I would very much like to get the bike operational again, but all I have is a 900W reciprocating saw. I had some moderate success with it and a metal blade, until the blade became dull after getting about 1 mm through the lock. Note that this was a very cheap blade, and I'm sure I can get some that are harder and more durable, but is it actually worth the effort and cost of blades to use a reciprocating saw for this? Or is an angle grinder a better tool that I should acquire instead?
For cutting locks, nothing beats an angle grinder. Good locks are all made with hardened steel that should be as hard as any metal cutting blade for a saw. The reciprocating saw will be able to make a little progress into the shackle, but its speed will dull the blade quickly, and once it's even a little dull, it will stop cutting and melt/grind the teeth completely off.
A grinder on the other hand works by heating the metal, softening it and scraping away particles. The grinding disk is meant to be consumable, so rather than getting dull, it just wears away as it does the job.
Of course, you should also consider that a lot of locks are very easy to pick open. While you are trying to get access to a grinder, spend a few minutes to research picking and determine the type of lock you have. It's unfortunate, but many locks claiming to be "high security" can be picked in minutes or seconds using improvised tools.
Although an angle grinder with a thin cut-off (type 1) wheel is the right tool; if you don't already have one then you can pay well over $70 for a good one.
You should look into diamond-grit or carbide-tipped blades per https://www.discountsawblade.com/Articles.asp?ID=264
If you dull down a section of the blade then you should be able to carefully continue cutting with a different section of blade since the reciprocating action only involves about 2 inches of movement and you have a 9 inch blade's worth of cutting material.
Additionally, if your reciprocating saw has an "Orbital" setting then make sure it is turned off or else your saw will be uncontrollable. Orbital is immensely useful for cutting wood though.
Get a torch and heat up a section until it turns cherry red and then allow it to cool. This will cause it to lose its temper and it will be soft enough to cut. You can then cut it with a reciprocating saw, angle grinder, or bolt cutters. If you use heat, protect the frame of the bike by wrapping it with a damp towel. You could also try finding a locksmith to pick the lock.