I heard running a halogen lamp on a dimmer will shorten its life. Is that true? I am asking as I have a Philips Halogená 150 w soft white bt15 in my bedside lamp on a dimmer.

I would not say shorten, but it wont prolong it as much as it does with inert gas filled incandescents.
How exactly it depends on the lamp construction and how exactly it is operated.

One significant wear mevhanism is filament recrystallization when locally overheating at power up (colder sections pose lower resistance, so hotter sections get overloaded; strong mainly for mains voltage lower power capsules). This effect would be likely completely suppressed, as the dimmer ramps up the power gradually, so the fulament has enough time to warm up evenly, so the local overloading is prefented.

Other effect is the evaporation being slower at lower temperature, so even when suppressed by the halogen cycle, this suppression is not 100% efficient (the tungsten settles to other hot components as well, not only the filament where it originated), so it means less of the residual tungsten loss.

Other effect, affecting mainly poorly designed lamps, is part of the filament being too cold, so instead of protecting it, the halogen cycle will attack it. Here the reduced power may actually cause shorter life, but such lamps usually suffer from this during wafm up and cool down cycles as well, so exhibit poor longevity anyway.

So if you are operating it in a reasonable range (where it really lights, not just being barely dark red filament), the life is not reduced.
Dont forget dimming any incandescent makes them extremely inefficient, so using kW lamps dimmed down to 10% brightness consumes 5x more power than using lamp with 10x lower output rating...

The problem that can happen when dimming a halogen lamp, is that the temperature would be too low to create the halogen cycle, as not all the halogen would be evaporated (In the case of bromine or chlorine), or sublimate (In the case of Iodine), so the filament would start sputtering.

Yes it will shorten the life of the bulb

It's not that simple, Mister pods. There are some power levels where the halogen cycle isn't active, but if the power level is low, the halogen cycle isn't needed anyway. Maybe 80% might decrease life a bit, but 20% will lengthen it substantially. However As noted above, this greatly decreases efficacy.

Really which effect is weaker and which stronger, so how and how much the resulting life changes with dimming, very heavily depends on the exact lamp and even fixture construction (include exact thermal balance, fill composition and pressures,...) and the point in the dimming range it is operated.
So one use pattern may prolong, some other shorten the life, or with the same patterns with one lamp model the life will be prolonged, with other shortened, and so on.

If you compare the cost of all effects, on the top of the list is energy consumption (result of the efficacy loss), then very long time nothing and only then, in the few percent range, are all the other effects.

So in my eyes if you are willing to pay the first (for the comfort the dimming gives,...; dont saying there is anything wrong with it, there are other comfort features which are way more "expensive" in our lives), there is not much point to argue about any of the rest anymore...

I run a Philips HalogenA 100W on a dimmer. Using a power meter I measured that at the level of light I feel is cozy it consumes about 50W, so pretty much half the rated power.
But I also often run it at full power when I need lots of light (otherwise I would just use a lower power lamp in the first place). This made me think whether running it on full power regularly would be sufficient to keep lamp life about the same rather than running it dimmed exclusively.

But as explained by Medved, it seems logical that the reduced filament temperature when dimmed outweights the drawback of inactive Halogen cycle anyways.

I run a Philips HalogenA 100W on a dimmer. Using a power meter I measured that at the level of light I feel is cozy it consumes about 50W, so pretty much half the rated power.

It is half of the rated power input, but barely 20% of the light output, so in the theory a 20W lamp would do the same business. So the 30W waste is there the price to pay (which becomes about 100kWAh over the 3kh life of the bulb) for that comfort.

Yep, I well aware of that. But a ~20W lamp would run the filament much hotter for the same light output and therefore the light would have a higher color temp, so not as cozy.

Nevertheless, I run the lamp about 1/3 of the time on full power, so I wouldn't say it's just for the cozyness but the comfort of having a great range of light levels depending on my needs at any given situation.