I would not use them, reason being the reason they were removed was they were approching the end of there expected life span.
Use 4 X 4's, 4 X 6's, or 6 X 6's. There all rated for below ground use.
I'd also pass on using landcape timbers. Read the tag on the ends of them, says right on the tag not rated for below ground use, what happens when you back fill, there now below ground and will rot out over time.
They were removed because they were approaching the end of their useful life as ties. In my area, most are removed as "spike kills", meaning they will no longer hold the spikes securely. I have corrals made from ties that the butts in the ground are still sound after 40 years. Most ties will last long after you get tired of them. In my opinion, pressure treated wood is very temporary when buried.