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Setting this issue to blocker without any discussion about the difficulty of implementation, and how that relates to the limited amount of time until the current feature freeze, is premature. I've set it to High.

Like Martin mentioned, PAL is just not designed to support this. My opinion is for the multiline options be disabled when curved placement is selected for v. 2.0, and implementation of this be moved to v. 2.1; but, only after other issues with curved labels are addressed, possibly offering a simpler means of supporting a multiline option.

I very much would like to have this feature as well, and have looked at many methods to do so. There are issues with implementing this, with regards to how PAL creates/handles curved labels, though I may have a solution.

PAL Issues¶

• Curvature offset - [see multiline-offset-comp.png] There are two approaches to offsetting the above/below lines in a multiline label, relative to the base line that PAL would create normally for one line: I refer to them as shifted and scaled in the png. An ideal implementation would use scaled, where the inside of curves are reduced on 'below' lines and expanded on 'above' lines. Unfortunately, until the issue with PAL not showing enough curved labels is fixed, this could result in label lines with tighter curves not showing up. If the whole label is to not be shown under that circumstance, then the tightest curve becomes a limiting factor. If the shifted approached is used, only one line in a multiline label will actually appear to follow a curve, and above/below lines may awkwardly intersect or overlap the underlying feature.

• Text alignment of offset - [see multiline-offset-comp.png] As you can see in the attachment, even small identical features that are manually offset via shifted or scaled methods do not exhibit consistent horizontal text alignment when output from PAL. There is currently no support in PAL for 'pinning' candidates to a particular point on a line (though I have an idea for that, see below).

• Number of lines - While ostensibly unrelated, supporting even-numbered multilines would mean also adding support to PAL for per-label distance offset (currently only per-layer is supported).

Some solutions¶

• My best solution so far - I have another idea for a feature I call 'label anchors' that could help with this. The defined anchor would tell PAL to only use one candidate and where it starts. (This is different than data defined x/y, which totally skips chosen placement algorithms and essentially treats the geometry as a point.) Then, the following could possibly be implemented:
  • Figure what the widest text line of a multiline label is and send it PAL as a single-line curved label.
  • Tag that label when sending into PAL as having above/below lines.
  • Adjust the height fontmetrics for the individual characters of that single line label to be similar to the final multiline label (so resultant label footprint can be part collision calculation).
  • After PAL solution, when labels are drawn, catch tagged multiline labels. Those labels are not drawn and are removed from list. Before removal, new geometries are generated based upon number of lines, offset according to their line height setting, and with geometry based upon the baseline of the single-line characters (or possibly a clipping from original geometry). Each new geometry is assigned the appropriate text relative to its line, and a calculated 'anchor' point that equates to its starting point according to how that text would be aligned relative to other lines (left, center, right).
  • After completion of drawing all other labels, a new PAL layer is created and the multiline geometries are registered with PAL. PAL is then solved for that single layer, which should produce labels correctly anchored to the provided line geometries.
  • Layer rendering execution is then, finally, returned back to vector layer.
  • (A similar approach can be used to create more generalized/simplified curved labels, offset from complex line features.)

• Qt implementation - It's possible to create text on a path with Qt. While this might allow handling of multilined labels outside of PAL, it would not afford collision resolution.

End of life notice: QGIS 2.18 LTR
Source:
http://blog.qgis.org/2019/03/09/end-of-life-notice-qgis-2-18-ltr/