SSAS Tabular Model or SQL Relational Database for new Datawarehouse? Is SSAS still a good to learn technology?
In my opinion, yes - it is still a technology that is completely valid. Even more so if we are discussing Tabular rather than Multi-Dimensional SSAS implementations. For example, VertiPaq (the database engine that runs Tabular Models) is the same engine that can be found in PowerBI and any skills you learn using SSAS/TOM/DAX will help you if you ever decide to learn that as well. Its also good to note that PowerBI, Excel, SSRS and many other tools can connect to SSAS/Tabular to obtain data to build dashboards and reports.

I personally have been using SSAS/Tabular since early 2014 and I have already rolled out 3 large-scale implementations (I'm currently working on my 4th). My current company uses SSAS/Tabular as the Analytics Engine for its core product. It can be integrated with many other products other than just SQL Server by using PowerQuery to obtain data from other sources.

If all you were planning to do was to have a data warehouse with a star schema (or even snowflake schema) then Tabular is a good choice as it offers fantastic performance (hardware dependent) and amazing compression. You should be aware though that it is a pure in-memory analytics engine, so if you choose to go down that route, be prepared to procure lots of RAM. To estimate how much RAM you will need, a typical Tabular installation will be 1/10th the size of the originating data (subject to good modelling principles). 

I have a Tabular Model which contains circa 800 million rows of data in the main fact table and around 16 million rows in each of the dimension tables. It takes up about 48GB of RAM, but queries against it are sub-second when applying a filter and less that 30 seconds when just dumping the entire table. Unlike SQL Server, partitioning in a Tabular Model does not speed up the query as the data is not on disk, but it does speed up your ETL process as you can load individual partitions.

With all that being said, I still have a SQL Server data warehouse which stores data that I don't want in my Tabular Model. As with any technology, choose the right tool for the job. If you want something that can give you high performance analytics Tabular is a great choice, but does require you to learn a few more technologies (C# &amp; PowerShell are a must in my opinion as they help with deployment, management and builds). However, if you don't have the budget, resources or need for the features that a Tabular installation offers, then a traditional data warehouse or a multi-dimensional SSAS installation may be the better way to go.