You are correct to wonder about much of the stuff you have read. Start with the term softwood? That is not really the true definition of the term but a term used by lumber folks to describe how easy or hard it is to cut the wood. How the tree reproduces or how hard to cut the wood might be is not a real factor in our use of the wood. So we need to move on to what we do need to know. One reason to move on is that when the wood is close to being ready for our use, it is going to be really, really hard to tell what species it comes from in many cases.
If we were speaking of the wood being hard or soft, that is a factor. Don't use wood that is already getting mushy as it won't last long. But then you probably had that in mind already! Don't use wood that has lots of sap left as that is where color or something harmful might originate. Pine and cedar are often mentioned and they are considered "softwoods" by lumber folks. But the problem for us is that they also tend to have lots of sap and retain that sap for a long time. It is the sap content, not the species which is important. In theory, the species might have some effect on how long wood lasts but then you are talking about 10 years vs. 20 and it goes out too far for me to worry about.
I like to look at what does matter. How much sap is left?
Starting with how flexible is good. Any bark left may lean toward being newer but not a final answer. Is the piece lighter feeling than expected for the size? Weight is a factor as water weights far more than dry cellulose. Cutting to get a close look at the inside gives good info. Wood dries from the outside to inside and it can be expected to change color as it dries so if the total piece is pretty close to uniform color all the way, it is likely to be dry.
Don't buy into the idea that softwoods are not good. I often use some really large pieces that I know to be cedar and they last plenty long enough.
This is what totally dry wood should look like when cut through. The wet edges are from the piece being soaked. The wet spots come from out to in when you do the bleach soak but that is not the color to look at for our purpose. Look how uniform the color down through the center is. But keep in mind that this is wood that has been dead and weathered for a VERY long time not just 5-10 years.
This is a cedar or juniper log that I picked. It was longer but the fellow who wanted it had hollow and shorter in mind so I tunneled it out and cut it down to what he wanted.
Dry cedar is safe to use.
Rainbow cichlids ( herotilapia multispinosa) protecting fry. Note the large cedar log.