OK some info to help everyone understand this. A lot of people use the term pass through in regards to their TV and ARC, eARC or optical connection. Pass through is technically not correct because of how HDMI works. If your source devices (cable box, streaming box, game console, disc player, etc.) are connected to the TV, the TV talks to the source device and the TV tells the source devices what it has for audio and video capabilities. The source device will only send what the TV can handle. If your TV doesn't have a dolby digital decoder (some older TVs don't) it will tell the source it can only handle stereo so send PCM only. It then can only pass through PCM via optical, ARC or eARC. So if the TV couldn't decode it it could only "pass through" PCM because that is all that the source would send.
ARC canonly handle DD, DD+, DTS and stereo PCM. It can not handle multi-channel PCM.
Optical is the same as ARC but can not handle DD+, but because DD+ is backward compatible with DD it still gives 5.1 sound.
eARC is better at communicating with the audio device and the will "pass through" formats it can't decode like Dolby True HD, DTS Master Audio, Multichannel PCM (ARC could only do stereo PCM) if the audio device is capable.
Now Atmos/DTS:X are different animals than DD and DTS. DD/DTS are channel based like 5.1, 7.1, 2.0, etc. Atmos/DTS:X are NOT channel based. Your receiver might be able to do 7.1.4 for four overhead speakers but the format is not encoded the same way. Atmos has information more like I want the sound to from the left from at 30 degrees to the back rear at 135 degrees and at what height. Think of Atmos/DTS:X like a bubble and it tells the processor where it wants the sound in the bubble and the processor figures out which combination of speakers to send it to. This is why Atmos/DTS:X is not converted to LPCM because they are not discrete channels like 5.1/7.1, etc. With higher end processors we can add more speakers and tell the processor where those speakers are in the bubble and it will figure out where to send the sound. For most receivers they just want you to put speakers in certain areas and it will process it based on those locations. This reduces cost in processing power.
So for your receiver or sound bar to get Atmos via ARC or eARC the TV must be able to pass through DD+. If an older TV it needs to be able to decode it as well or it likely will not be requested from the source. Most streaming sources use DD+ and not Dolby True HD due to bandwidth. It is not lossless sound like Blu-ray but still enjoyable, Blu-ray is really the only source that has the lossless formats.